Gene Definition Evolution and Comparative Genome Structure

The Evolving Concept of the Gene

The concept of the gene has varied throughout history as knowledge of its function and structure has increased. Initially, the term was applied to hereditary factors studied by Mendel.

Later in the twentieth century, the gene was localized to the chromosome (the chromosomal theory of inheritance of the early twentieth century).

In the 1940s, in accordance with the one gene–one enzyme principle, it was considered that the gene is a region of the genome that directs the synthesis of a single enzyme. This theory was proposed by Beadle and Tatum, whose experiments using the fungus Neurospora demonstrated this relationship. Following this work, a gene was defined as a piece of DNA that encodes a polypeptide chain.

In the late 1960s, the discovery of fragmented genes again modified the concept of the gene, which was then considered a set of DNA sequences, distinguishing between the following components:

• Coding Sequences: These sequences (exons) are translated into proteins. They are not continuous in the DNA but are divided into fragments separated by non-coding sequences (introns).
• Untranslated Sequences: These include the promoter region where RNA polymerase binds.
• Regulatory Sequences: These include enhancer or silencer elements located far from the gene that regulate the expression of one or more genes.

Finally, due to RNA maturation (specifically, alternative splicing), some DNA sequences can encode more than one protein with different biological functions. Despite these complexities, the core definition of a gene encoding a polypeptide chain is often maintained.

Genome Structure

Prokaryotic Organisms

The genome of prokaryotic organisms consists primarily of a single circular chromosome. Prokaryotic genes are typically continuous (the entire genetic information contained in each gene results directly in the formation of a protein).

Many bacteria also contain plasmids—circular DNA molecules smaller than the main chromosome, possessing the ability to replicate independently.

Eukaryotic Organisms

In eukaryotes, the majority of DNA is found in the nucleus, forming the nuclear genome. The eukaryotic genome is significantly more complex due to several factors:

• A much larger amount of DNA.
• The presence of repetitive DNA sequences.
• Genes are fragmented into portions: introns and exons.
  • Introns are transcribed but not translated.
  • Exons are transcribed and translated, meaning they contain the information necessary to form a polypeptide chain (encoding the amino acid sequence).
• DNA is associated with proteins, specifically histones, to form chromosomes.

A portion of the eukaryotic genome is also found in organelles, specifically chloroplasts and mitochondria (organellar genomes).